Apparatus for repairing hybrid circuits

ABSTRACT

A fixture for use in repairing hybrid circuits which has an arm pivotally mounted on a pair of vertical shafts near the lower ends thereof that are secured to a base, a horizontal upper plate being rigidly secured to the shafts adjacent their other ends. The upper plate and a screen that fits over the former have openings therein that are in alignment. A heater assembly is mounted orthogonal to the arm near the end thereof on one side of the pivot point. A cable is connected between a foot pedal and the end of the arm on the other side of the pivot point. The foot pedal is pressed to pivot the arm, and thus the heater assembly, to move the tip end of the latter into the opening in the screen and into contact with the underside of a hybrid circuit substrate that is placed on top of the screen. When the solder joints on a particular selected defective component are observed to melt, the foot pedal is released and the component removed from the substrate. The tip of the heater assembly comprises an aluminum shank that is press fit into the bore in one side of the stainless steel cap, the surface on the other side of the cap being shaped like a spherical cap. The aluminum shank is chemically more active than and operates as a reducing agent on the steel (iron) cap to maintain only a thin film of oxide on the latter. The surface of the spherical cap has only a small area thereof that is in physical contact with a hybrid substrate. The remainder of the cap surface is in close proximity to the substrate, however, so that the latter is heated by both conduction and radiation.

United States Patent Daebler et al.

[ Sept. 9, 1975 APPARATUS FOR REPAIRING HYBRID CIRCUITS [75] Inventors: Donald H. Daebler; Richard P.

Malmgren. both of Albuquerque. N. Mex.

[73} Assignee: GTE Automatic Electric Laboratories Incorporated, Northlake. Ill.

[22] Filed: Dec. 26. 1973 [2]] App]. No.: 427.617

[52] L'.S. Cl .l 228/8; 228/32 [Sl] Int. Cl. 523K 1/00 [58] Field of Search 228/8. 25, 32, 45; 29/575. 29/590. DIG. 13. DIG. 24. 473.1. 472.7. 472.5. 484

[56] References Cited UNITED STATES PATENTS l.-$89.l53 4/1924 Robinotf 228x32 Primary Examiner-Al Lawrence Smith Assistant Examiner-Margaret M. Joyce Aiwrney. Agent. or Firm-Russell A. Cannon; Leonard R. Cool 57 ABSTRACT A fixture for use in repairing hybrid circuits which has an arm pivotally mounted on a pair of vertical shafts near the lower ends thereof that are secured to a base. a horizontal upper plate being rigidly secured to the shafts adjacent their other ends. The upper plate and a screen that fits over the former have openings therein that are in alignment. A heater assembly is mounted orthogonal to the arm near the end thereof on one side of the pivot point. A cable is connected between a foot pedal and the end of the arm on the other side of the pivot point. The foot pedal is pressed to pivot the arm. and thus the heater assembly. to move the tip end of the latter into the opening in the screen and into Contact with the underside of a hybrid circuit substrate that is placed on top of the screen. When the solder joints on a particular selected defective component are observed to melt. the foot pedal is released and the component removed from the substrate. The tip of the heater assembly comprises an aluminum shank that is press fit into the bore in one side of the stainless steel cap. the surface on the other side of the cap being shaped like a spherical cap. The aluminum shank is chemically more active than and operates as a reducing agent on the steel (iron) cap to maintain only a thin film of oxide on the latter. The surface of the spherical cap has only a small area thereof that is in physical contact with a hybrid substrate. The remainder of the cap surface is in close proximity to the substrate. however. so that the latter is heated by both conduction and radiation.

21 Claims, 6 Drawing Figures PATENTED 35? 91975 SHEET 1 BF 6 PATENTED SEP 9 sum 2 BF 6 PATENTED 9% sum 3 or gs FIG.

sum 5 0F 6 PATENTED SEP 91975 APPARATUS FOR REPAIRING HYBRID CIRCUITS BACKGROUND OF THE INVENTION This invention is related to equipment for reworking electrical circuits and more particularly to a fixture for repairing hybrid circuits.

Hybrid circuits are made up ofa plurality of discrete components that are mounted and interconnected on a glass or ceramic substrate. These hybrid circuits are primarily manufactured by either thin-film or thickfilm techniques. In view of the recent advances in the thin-film and thick-film technologies, hybrid circuits manufactured by these techniques are finding increased application. It is desirable to be able to repair an assembled hybrid circuit by removing a defective discrete component or integrated circuit package and replacing it with an operative unit rather than have to scrap the whole hybrid circuit.

Prior-art techniques for repairing hybrid circuits include: using a hand soldering iron; and using a rework fixture employing a hydrogen flame as a heating mechanism. When a soldering iron is used to remove a defective component, the hot tip of the iron is sequentially placed directly on each connection of a component lead to a conductive pattern on a substrate while the component is twisted to separate that lead from the pattern. Such an operation requires a skilled operator in order that this direct application of heat to the conductive pattern and component lead and this twisting of the component not result in irreparable damage to the conductive patterns and thus to the hybrid circuit. In the alternate prior-art technique, a concentrated hydrogen flame is sequentially applied to the underside of the substrate opposite each connection of a component lead to a conductive pattern. This technique also requires twisting of the component as the substrate opposite each lead is sequentially heated. If the flame is left in one spot in an attempt to heat an area of the ceramic substrate, irreparable damage may be caused to a conductive pattern. Also, the thermal shock of initial application of the concentrated hydrogen flame may crack the substrate.

An object of this invention is the provision of im proved method and apparatus for repairing hybrid circults.

DESCRIPTION OF DRAWINGS This invention will be more fully understood from the following detailed descriptions of preferred embodiments thereof, together with the drawings in which:

FIG. 1 is a right side elevation view of apparatus embodying this invention;

FIG. 2 is a front elevation view of the apparatus in FIG. 1;

FIG. 3 is a top view of the apparatus in FIG. 1;

FIG. 4 is a right side elevation view of an alternate embodiment of this invention;

FIG. 5 is a front elevation view of another alternate embodiment of this invention; and,

FIG. 6 is a front elevation view of yet another embodiment of this invention. The elements in the various figures are not necessarily drawn to scale. Also, the II lustration in FIGS. 46 are not drawn to the same scale as are the illustrations in FIGS. 1-3. Similar elements in the various figures are designated by the same or primed reference characters.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to FIGS. I3, a preferred embodiment of this invention comprises a base member 4, and upper and lower plates 5 and 6, respectively, that are rigidly attached to a pair of posts 7. An arm 8 is pivotally mounted on the posts '7 by locating a shaft 9 in the arm and attaching the shaft to the posts 7 by nuts 10, see FIG. 2. The U-shaped member 4 is made of a heavy material such as cast iron to provide a weighted base for the fixture. A plurality of rubber foot pads 11 are mounted in the bottom of the base 4 to keep the fixture from sliding on a work table.

The arm 8 is a channel-shaped member having holes 12 in the side walls thereof, see FIG. 2. The side holes 12 are located approximately three quarters of the length of arm 8 from the one end 14 thereof. The dimensions of the center and end sections of shaft 9 are sized to slide smoothly within the side holes 12 in the arm 8 and the holes 15 in the posts '7, respectively. Arm 8 is pivotally mounted between posts 7 by locating the center and end sections of shaft 9 in the openings in the arm and posts, respectively, and attaching nuts 10 to the ends of the shaft.

A heater assembly 18 is mounted near the end 14 of arm 8. The heater assembly in FIGS. 1 3 is essentially a vertically mounted special-purpose soldering iron with a special tip 21 thereon, the latter comprising the shank 28 and cap 29. The curved surface of cap 29 is shaped to heat the substrate 58 of a hybrid circuit 57 in FIG. 2 by both conduction and radiation as is described more fully hereinafter.

The heater assembly 18 comprises a central heater element 19 having an internally wound electrical heating coil (not shown) over at least a portion of the length thereof, a bore 20 in the upper end of element 19, a heat transfer tip 21 in the bore 20, and a mounting bracket 22 that is rigidly secured, e.g., by silver soldering or welding to the lower end of element 19. The heater element 19 may, by way of example, be the cylindrical heater element section of a standard watt soldering iron. The bracket 22 is secured to the top wall of arm 8 by screws 23 and nuts 24 so as to mount the heater element 19 orthogonal to the plane of the arm 8. A cord 25 from the lower end of element 19 extends through the hole 26 in the arm 8 and the channel section thereof to a power source (not shown) for heating the element 19 and thus the tip 21. Alternatively, a torch could be attached to arm 8 with a hydrogen flame, for example, directed onto the upper end of element 19 for heating the tip 21.

The heat transfer tip 21 comprises a shank 28 and cap 29 having a bore 30 therein. The shank 28 and cap 29 are preferably made of metals that have good thermal conductivity characteristics. One end of shank 28 is press fit into the bore 30 of cap 29. The other end of shank 28 is secured in the bore 20 of heater element 19 by an Allen screw 31. The portion of cap 29 opposite bore 30 is preferably a spherical cap such as would be formed by removing a truncated section of a hemisphere from the latter. The curved side of cap 29 may also be a right section of a cone or paraboloid. Alternatively, the curved surface of cap 29 may have a flat formed on the center of the surface thereof opposite the bore 30 therein that is parallel to and contacts a substrate 58, with the cap surface receding gradually downward away from the flat. In practice, such a cap 29 may be a truncated section of a spherical cap. a truncated cone. or a truncated paraboloid.

The shank 28 is preferably made of a material that is chemically more active than the material of cap 29. By way of example. the shank 28 may be made of 6061-T6 aluminum and the cap 29 made of 303 stainless steel. The surface of cap 29 will oxidize during operation of the unit. Since this surface oxide is a heat buffer. it will impair and eventually may inhibit heating ofa substrate material brought into contact with it if the thickness of the oxide layer is allowed to increase. It has been found. however. that a substrate may be damaged if a nonoxidized cap surface that has been tinned is brought in contact with it. Thus. it is desirable to maintain a thin oxidation layer on the contact surface of the cap. This is accomplished in accordance with this invention by the chemically more active aluminum shank 28 operating as a reducing agent on the chemically less active stainless steel cap 29 to prevent the surface of the latter oxidizing past the point of a thin film of oxide.

The plate 6 is placed over the lower ends of the posts 7 before the latter are secured to base 4 by nuts 32. which are located in associated recesses 33, see FIG. 2. Plate 6 is preferably made of a shock absorbent material such as Delrin. The end 34 of the lower plate 6 extends a sufficient distance into the opening between the feet of base plate 4. see FIG. 3. so as to limit the travel of the one end 14 of arm 8 so that it cannot contact the top of a table on which the fixture is operated. e.g.. if the arm 8 is dropped. Alternatively. a bar (not shown) between the feet of the base 4 may be employed in place of the plate 6.

The upper plate 5 is a U-shaped member that is preferably made of a light weight material such as Delrin. The plate 5 is secured onto the upper ends of posts 7 by nuts 35. A screen 36 is placed over the top. front. and two sides of plate 5 as is shown in FIGS. 1 and 2 to prevent an operator from inadvertently contacting the heater assembly when parts thereof are hot. The screen 36 is secured to plate 5 by screws 38. The screen 36 is preferably a 0.040 inch thick aluminum sheet having perforations therein that are located on a onequarter inch grid. The aluminum sheet is hard-coat anodized to provide a tough surface on which to locate and move a ceramic substrate. Alternatively. screen 36 may be made ofa stiff wire mesh. A generally rectangular opening 39 that is slightly larger than the diameter of cap 29 is formed in the top surface of screen 36. see FIG. 3. with the wires 41 and 42 bisecting the opening 39. The screen 36 is positioned on the upper plate 5 to align the center of cap 29 with the wires 41 and 42 prior to tightening the screws 38. The relative positions of the opening 39 and cap 29 are located so that when the arm 8 is pivoted about shaft 9. the cap 29 moves into the center of the opening 39.

A cable assembly that is associated with a foot pedal 45 is employed to pivot the arm 8 to move the cap 29 into the opening 39. A cable assembly that is useful in this application is the Flexline foot pedal with associated cable and connections that is manufactured by Hughes Aircraft Corporation. The cable assembly comprises an inner cable 46. a cable sheath 47. a pulley 48 that is mounted on a support plate 49. and a cable stop 50. One end of the inner cable 46 and sheath 47 are integrally connected to the foot pedal 45. The other end of sheath 47 is rigidly secured to plate 49, which is attached to the other end of plate 6 by screws 51 and nuts 52. Alternatively. the plate 49 can be connected directly to the base 4 and posts 7. A spring 53 is positioned over the threaded end of bolt 54 so that the spring is under compression when the arm 8 is parallel to the upper plate 5. The other end of the inner cable 46 is threaded through pulley 48 and the hole 55 in arm 8. The cable stop 50 is attached to the free end of inner cable 46 that is immediately adjacent the top surface of arm 8. When the foot pedal 45 is depressed. the inner cable 46 is withdrawn so that arm 8 pivots about shaft 9. With the pedal 45 depressed. the position of shank 28 in the bore 20 of the heater element 19 is adjusted to make the end of cap 29 be approximately 0.02 inch above the top surface of screen 36. This adjustment may be accomplished by laying a metal strip having a thickness that is equal to that of the screen 36 across the arms of the upper plate 5 prior to attaching the screen 36 to the plate 5. The height of cap 29 is varied by loosening the Allen screw 31 and manually raising or lowering the shank 28 in bore 20.

A typical hybrid circuit 57 that is to be repaired with the fixture in FIGS. 1-3 may be made by thick-film techniques. Such a thick-film circuit includes discrete components that are soldered to conductive patterns formed on. for example. a ceramic substrate 58. In order to remove a defective component and replace it with a new one. hybrid circuit 57. see FIG. 2, is placed over the opening 39 in screen 36, see FIG. 3. The circuit 57 is positioned on screen 36 with the side of substrate 58 supporting the discrete components facing away from screen 36. The defective component of the hybrid circuit 57 is centered over the opening 39 in the screen. This alignment is accomplished by centering the defective component with the intersection of imagi nary lines through wires 41 and 42.

In order to remove the defective component that is centered over the opening 39 in screen 36. the cord 25 is connected to a source of AC power to heat up the cap 29 of the heating element. When cap 29 is hot. the foot pedal 45 is depressed to pivot arm 8 to move the cap 29 into contact with the flat bottom surface of the substrate 58. Since the surface of cap 29 is essentially spherically shaped. only a point or small area of the cap actually contacts the substrate 58 and heats the latter by conduction. In accordance with a preferred embodiment of this invention. the radius of curvature of the surface of the spherical cap 29 is large with respect to the cord length d thereof in FIG. 2. This means that in the area surrounding the point of contact between the substrate 58 and cap 29, the surface of the cap is very close to the substrate so that the latter is also heated by radiation from the cap. This operation enables the use of a lower temperature heater element 19 and cap 29 in this fixture. and thus reduces the thermal shock in the substrate 58 when the hot heater cap 29 is brought into contact with it. When the solder junctions of the defective component are observed to melt. the foot pedal 45 is released and the component removed from the substrate. e.g.. with tweezers. A new component is mounted on the substrate by fluxing associated solder joints. pressing the new component into the flux. and holding the foot pedal 45 depressed until the solder pads are observed to reflow.

This technique has been used in ref'lowing a single connection and in removing discrete component packages with as many as 14 connections. In a fixture that was actually built and tested. the radius of curvature of the spherical cap 29 was 3 inches, and the diameter d of the base of the cap was 0.375 inch. It was found to require only approximately 2 seconds of contact time between the cap 29 and a ceramic substrate 58 to reflow a single connection and only approximately 5 seconds to flow the solder associated with an eight-lead package. It only requires an additional time interval of contact between the cap 29 and a substrate 58 to flow the solder connections of large multi-lead components over a single pair of leads. It has been found that this longer heating interval does not damage the substrate or thick-film conductive patterns formed thereon.

An alternate embodiment of this invention in which movement of the heater assembly 18 is produced by an eccentric cam 61, rather than by the pivoting arm 8, is shown in FIG. 4. The heater element 19 is coaxially located in a sleeve 62 with the longitudinal axes AA thereof orthogonal to the plane of the base 4'. The sleeve 62 is partially broken away in FIG. 4 in order to more clearly illustrate the parts therein. The heater element 19 and sleeve 62 are supported in an upright position with the lower end of element 19 contacting the periphery of the cam 61 by support members 63 and 64 having opposite ends thereof attached to the base 4' and the cylindrical flange 65 that is secured to the periphery of sleeve 62. The sleeve 62 is supported by a plurality of members 63 and 64, only two of which are shown in FIG. 4. A cylindrical compression spring 66 (only one-half of which is shown here for the sake of clarity) is located in the space between the periphery of element 19 and the inner surface 67 of the sleeve 62. The spring is maintained in compression between the shoulder 68 on sleeve 62 and the opposing shoulder of a ring 69 on element 19 near the lower end thereof to keep this end of element 19 in contact with the periphery of cam 61. The ring 69 is attached to element 19, c,g., by Allen screws. The screen 36, upper plate 5', and posts 7' are mounted on base 4' as was described in relation to the embodiment ofthis invention in FIGS. l3. The height of the cap 29 on the heater element is adjusted as described previously, with the cam 61 rotated to produce a maximum eccentricity in the positive vertical direction.

The cam 61 is driven by a belt 71 which extends between a drive pulley 72 on a motor 73 and a driven pulley 74 on the cam 61. Alternatively, a drive chain connected between a pair of sprockets may be used in place of the belt 71 and pulleys 72 and 74. The lines 75 and 76 from motor 73 are connected through a control circuit 77 to a power source 78. The control circuit is responsive to actuation of switch 79 for: energizing motor 73 to selectively produce a 180 rotation of cam 61; tie-energizing motor 73 for a prescribed time interval; and again energizing motor 73 to produce another 180 rotation of cam 61. The circuit 77 then maintains the drive motor 73 inoperative until the next time switch 79 is actuated.

Prior to operating the assembly in FIG. 4, the cam 61 is adjusted to provide a maximum eccentricity in the negative vertical direction and the cord 25 is connected to a power source such as source 78 to heat the cap 29. When the cap 29 is hot and a hybrid circuit substrate is positioned over the opening 39 in screen 36, switch 79 is actuated to cause circuit 77 to energize motor 73 to rotate the cam [80 to move cap 29 into contact with the underside of the substrate. After a selected prescribed time interval for the solder to flow, control circuit again automatically actuates motor 73 to drive cam 61 to its position of maximum downward vertical eccentricity. The compression spring 66 expands to move element 19 downward and cap 29 away from the substrate. An alternate method in which an operator selectively maintains the cap 29 on the heater element 19 in contact with the substrate employs a latching mechanism (not shown) in cooperation with a cylindrical flange at the top edge of element 19. As the cam 61 rotates past its upper eccentric position, the latch catches the flange to hold element 19 in its uppermost position with cap 29 contacting the substrate. The cam 61 is stopped in a position other than at the point of maximum positive vertical eccentricity. After the solder is observed to melt, the latch is selectively released by the operator to allow spring 66 to move cap 29 away from the substrate.

The embodiment of this invention in FIG. 5 is similar to that shown in FIG. 4, except that a solenoid 81 is employed to provide the vertical movement of heater element 19 instead of the eccentric cam 61. The heater element 19 in FIG. 5 is aligned by and slides smoothly in the bore 82 of the solenoid 81. The element 19 and solenoid 81 are suspended above base 4 in an upright position by a support frame 83 having feet attached to the base and having a flat platform 84 attached to 3. cylindrical flange 85 that is secured to the periphery of solenoid 81. A sleeve 86 fits over and is attached to the other end ofsolenoid 81, e.g., by Allen screws. The solenoid 81 and sleeve 86 are partially broken away in FIG. 5 in order to more clearly illustrate the parts located therein.

A sleeve 87 having an outwardly projecting cylindrical flange 88 adjacent one end thereof is attached to the periphery of element 19 near the top end thereof, e.g., by Allen screws. The sleeve 86 has an inwardly projecting cylindrical flange 89 on the upper end thereof. A spring 90 (only one-half of which is shown here for the sake of clarity) is maintained in compression between the opposing surfaces of flanges 88 and 89 for moving element 19 to its lowest position in the solenoid, with the flange 88 contacting the end 91 of the solenoid when the latter is not energized. The end 92 of sleeve 87 contacts the flange 89 to limit vertical movement of sleeve 87 and thus cap 29 when the solenoid is energized. The windings of solenoid 81 are con nected through lines 95 and control circuit 96 to a source 97 of DC current.

In order to adjust the heater assembly for the maximum desired height of cap 29, the element 19 is raised to its maximum height in the solenoid with the end 92 of sleeve 87 contacting the lower surface of flange 89. The height of cap 29 is then adjusted as previously described in relation to the embodiment of the invention in FIGS. 1 3. After locating screen 36 on plate 5', the wires 25 are connected to an AC power source to heat cap 29.

In operation, a hybrid circuit substrate is positioned over the hole 39 in screen 36. The switch 98 is actuated to cause control circuit 96 to pass a DC electric current through the windings of solenoid 81 to create an axial magnetic field therein that provides a vertical movement of element 19. This operation moves the hot cap 29 against the lower surface of the substrate when the end 92 of sleeve 87 contacts the lower surface of flange 89. After a selected prescribed contact time interval for the solder on the substrate to flow. control circuit 96 disconnects source 97 from the solenoid. This causes the field produced thereby to collapse and spring 90 to move element 19 downward to its equilibrium position, with the surface of flange 88 contacting the end 91 of the solenoid. Alternatively, the direction of the DC cur rent through solenoid 81 may be reversed to return element 19 to its equilibrium position. Also, a ring of a compressible resilient material may be placed over the end 92 of sleeve 87 to make the movement of cap 29 into contact with the substrate more gradual.

Another embodiment of this invention is illustrated in FIG. 6 wherein a hydraulic assembly is employed to effect the desired vertical movement of heater element 19 and cap 29. The hydraulic assembly in FIG. 6 comprises a right circular cylinder 101 supporting the element 19 in the bore 102 thereof, a master cylinder 103, a pressure source 104 of gas, a pair of valves 105 and 106, and a control circuit 107 having a switch 108 associated therewith. The cylinder 101 is suspended above the base 4' in an upright position by a support frame 83' having feet attached to base 4' and having a flat platform 84' attached to a cylindrical flange 85' that is secured to the lower end of the periphery of cylinder A sleeve 86 fits over and is attached to the other end of cylinder 10] (e.g., by Allen screws), the cylinder 101 and sleeve 86 being partially broken away in FIG. 6 to more clearly illustrate the parts therein. A compression spring 90 is supported between opposing surfaces of the cylindrical flanges 88 and 89 as was described in relation to the embodiment of this invention illustrated in FIG. 5.

A chamber 111 is formed between the lower end 112 of element 19 and the walls of cylinder 101. The wires 25 from element 19 extend through a tube 114 that is secured to the end 112 of the heater element and through the bottom of cylinder 101. O-rings 115 and 116 are located around the tube 114 and element 19, respectively, to provide seals between the associated moving parts.

The master cylinder 103 has first and second chambers 117 and 118 therein that are defined by a piston 119. The piston is supported in cylinder 103 by a shaft 121 on the former piston and the O-ring seals 122 and 123. The chambers 111 and 117 are filled with a noncompressible fluid such as oil and are connected together by a tube 124. Alternatively, chambers 111 and 117 may be filled with deionized water. The second chamber 118 in master cylinder 103 is connected through the input valve 105 to the pressure source 104, and through the output valve 106 to the atmosphere. The output of control circuit 107 on lines 125 is connected to each of the valves for controlling the operation thereof. The maximum travel of element 19 and the maximum height of cap 29 are initially adjusted by closing valve 106 and opening valve 105 to increase the pressure in the second chamber 118 to a prescribed value. The height of cap 29 is then adjusted to be that necessary to make it contact the underside of the substrate ofa hybrid circuit on screen 36. The input valve 105 is then closed and output valve 106 opened to move cap 29 away from the substrate to its equilibrium position.

In operation, the wires 25 are connected to an AC power source to heat the cap 29 and the substrate of a 6- hybrid circuit is positioned over the opening 39 in the top of screen 36. When the cap 29 is hot, the pushbutton switch 108 is pressed to activate circuit 107 which produces a signal on line 125 that causes valve 106 to close and valve 105 to open in order to pressurize the chamber 118 at a prescribed value. This causes piston 119 to move to the right in FIG. 6 in order to move cap 29 vertically and into contact with the sub strate. After the selected-prescribed time interval that is required to melt the solder at particular terminals on the substrate, circuit 107 automatically produces a signal on line 125 that causes valve 105 to close and valve 106 to open in order to exhaust chamber 118. The energy in the compressed spring then returns the element l9, and thus piston 119, to its equilibrium position with cap 29 spaced from the substrate on the screen 36.

Alternatively, the selective opening and closing of valves and 106 may be under the control of the operator by modifying the circuit 107. In this instance where the operator is in control, when switch 108 is pressed the first time, a signal from circuit 107 causes valves 106 and 105 to close and open, respectively, to move cap 29 against the substrate. When the operator notes that the solder at particular connections is melted, switch 108 is actuated a second time to cause circuit 107 to produce a signal on line that opens and closes the valves 106 and 105, respectively, to release the pressure in chamber 118 and allow cap 29 to move away from the substrate. Also, a current source 126 and solenoid 127 that encompasses the shaft 121 on piston 119 may be employed to control movement of the latter, and thus cap 29, in both directions instead of valves 105 and 106 and the spring 90 and associated flanges 88 and 89. In this implementation of the invention, current is passed through the windings of solenoid 127 in one direction to move the piston 119 to the right, and in the opposite direction to move the piston to the left in FIG. 6.

We claim: 1. Apparatus for operation in conjunction with a source of energy for reworking a hybrid circuit having discrete components solder mounted on one side of a substrate of the circuit, comprising heater assembly means comprising a cap and first shank that are made of material having good thermal conductivity characteristics, said cap having a bore therein receiving one end of said first shank in a heat-transferring relationship, said cap being made of a metal that is chemically less active than the metal from which said first shank is made for maintaining a thin film of oxide on the surface of said cap; first means for communicating said heater assembly means with the energy source for heating said cap;

second means for supporting the substrate of the hybrid circuit in a substantially horizontal position with the underside thereof facing downward adjacent said cap; and

third means for moving said cap into contact with the underside of the substrate that is supported by said second means for heating solder on the one side of the substrate through the latter.

2. Apparatus according to claim 1 wherein the surface of said cap contacting the underside of the substrate is shaped to provide both conduction and radiation heating of an associated area of the underside of the substrate.

3. Apparatus according to claim 2 wherein the cap surface contacting the substrate is shaped like the surface of a spherical cap.

4. Apparatus according to claim 2 wherein the cap surface contacting the substrate is shaped like the surface of a paraboloid.

5. Apparatus according to claim 2 wherein the cap surface contacting the substrate is shaped like the surface of a cone.

6. Apparatus according to claim 2 wherein the cap surface contacting the substrate has a flat formed on the center thereof, said surface receding gradually downward away from said flat.

7. Apparatus according to claim 6 wherein the cap surface contacting the substrate is shaped like the sur face of a truncated cone.

8. Apparatus according to claim 6 wherein the cap surface contacting the substrate is shaped like the surface of a truncated spherical cap.

9. Apparatus according to claim 6 wherein the cap surface contacting the substrate is shaped like the surface of a truncated paraboloid.

10. Apparatus according to claim 2 wherein the energy source is a source of electrical power and wherein said heater assembly means comprises a second cylindrical shank that is electrically heated from the energy source and has a bore in one end thereof, the other end of said first shank being located in the bore of said second shank for heating said cap.

11. Apparatus according to claim 10 wherein said third means comprises a support member,

an arm pivotally attached to said support member,

fourth means attaching said second shank to said arm, and,

fifth means for pivoting said arm about the connection thereof to said support member to move said cap into contact with the underside of the substrate.

12. Apparatus according to claim 11 wherein said arm is attached to said support member at a point intermediate the ends of the former arm, and said second shank is mounted substantially orthogonal to said arm near one end thereof with said cap spaced above said arm; said third means including a selective actuation mechanism and a cable connected between said actua tion mechanism and the other end of said arm, actuation of said mechanism pulling the other end of said arm for pivoting said cap into contact with the underside of the substrate.

13. Apparatus according to claim 12 wherein said actuation mechanism is a foot pedal and the other end of said arm is spring loaded for moving said cap away from the substrate when said foot pedal is released.

14. Apparatus according to claim 10 wherein said third means comprises an eccentric cam for moving said second shank in a substantially vertical plane for moving said cap into contact with the underside of a substrate.

15. Apparatus according to claim 10 wherein said third means comprises a solenoid for moving said second shank in a substantially vertical plane for moving said cap into contact with the underside of a substrate. 16. Apparatus according to claim 19 wherein said third means comprises hybraulic means for moving said second shank in a substantially vertical plane for moving said cap into contact with the underside of a substrate.

17. Apparatus according to claim 10 wherein said third means comprises a pneumatic mechanism for moving said second shank in a substantially vertical plane for moving said cap into contact with the underside of a substrate.

18. Apparatus according to claim 10 wherein said third means comprises a control circuit responsive to selective actuation thereof for moving said cap into contact with the underside of the substrate for a predetermined time interval and automatically withdrawing said cap from the substrate after said time interval is over.

19. Apparatus for operating in conjunction with a source of energy for reworking a hybrid circuit having a discrete components solder mounted on one side of a substrate by performing one of the operations of removing a discrete component from or attaching it to the substrate, said apparatus comprising:

heater assembly means comprising a cap and a shank that are made of material having good thermal conductivity characteristics, said cap having a bore therein receiving one end of said shank in a heat transferring relationship, said cap being made of a metal that is chemically less active than the metal from which said shank is made for maintaining a thin film of oxide on the surface of said cap; first means for communicating said heater assembly means with the energy source for heating said cap;

second means for supporting the substrate with the other side thereof, that is opposite the one side of the substrate, being adjacent said cap; and

third means cooperating with said heater assembly means and said second means for providing relative movement of said substrate and said cap for causing a portion of said cap surface and an area on the other side of the substrate to come into contact with each other for heating through the substrate solder on the other side thereof.

20. Apparatus according to claim 19 wherein the portion of said cap surface contacting the other side of the substrate is shaped to provide both conduction and radiation heating of a region of the substrate that extends over a surface area thereof that is greater than the Contact area on the other side of the substrate,

21. Apparatus according to claim 20 wherein said third means moves said cap into contact with the other side of the substrate.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,904,100

DATED September 9, 1975 INVENT0R($ 1 Donald H. Daebler & Richard P. Malmgren It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 10, in claim 16, "according to claim 19" should read according to claim 10 Signal and Scalccl this Arrest:

RUTH C. MASON Arresting Officer C. IARSIIALI. DAMN Commissioner nfflnenu' and Trldeullrks 

1. Apparatus for operation in conjunction with a source of energy for reworking a hybrid circuit having discrete components solder mounted on one side of a substrate of the circuit, comprising heater assembly means comprising a cap and first shank that are made of material having good thermal conductivity characteristics, said cap having a bore therein receiving one end of said first shank in a heat-transferring relationship, said cap being made of a metal that is chemically less active than the metal from which said first shank is made for maintaining a thin film of oxide on the surface of said cap; first means for communicating said heater assembly means with the energy source for heating said cap; second means for supporting the substrate of the hybrid circuit in a substantially horizontal position with the underside thereof facing downward adjacent said cap; and third means for moving said cap into contact with the underside of the substrate that is supported by said second means for heating solder on the one side of the substrate through the latter.
 2. Apparatus according to claim 1 wherein the surface of said cap contacting the underside of the substrate is shaped to provide both conduction and radiation heating of an associated area of the underside of the substrate.
 3. Apparatus according to claim 2 wherein the cap surface contacting the substrate is shaped like the surface of a spherical cap.
 4. Apparatus according to claim 2 wherein the cap surface contacting the substrate is shaped like the surface of a paraboloid.
 5. Apparatus according to claim 2 wherein the cap surface contacting the substrate is shaped like the surface of a cone.
 6. Apparatus according to claim 2 wherein the cap surface contacting the substrate has a flat formed on the center thereof, said surface receding gradually downward away from said flat.
 7. Apparatus according to claim 6 wherein the cap surface contacting the substrate is shaped like the surface of a truncated cone.
 8. Apparatus according to claim 6 wherein the cap surface contacting the substrate is shaped like the surface of a truncated spherical cap.
 9. Apparatus according to claim 6 wherein the cap surface contacting the substrate is shaped like the surface of a truncated paraboloid.
 10. Apparatus according to claim 2 wherein the energy source is a source of electrical power and wherein said heater assembly means comprises a second cylindrical shank that is electrically heated from the energy source and has a bore in one end thereof, the other end of said first shank being located in the bore of said second shank for heating said cap.
 11. Apparatus according to claim 10 wherein said third means comprises a support member, an arm pivotally attached to said support member, fourth means attaching said second shank to said arm, and, fifth means for pivoting said arm about the connection thereof to said support member to move said cap into contact with the underside of the substrate.
 12. Apparatus according to claim 11 wherein said arm is attached to said support member at a point intermediate the ends of the former arm, and said second shank is mounted substantially orthogonal to said arm near one end thereof with said cap spaced above said arm; said third means including a selective actuation mechanism and a cable connected between said actuation mechanism aNd the other end of said arm, actuation of said mechanism pulling the other end of said arm for pivoting said cap into contact with the underside of the substrate.
 13. Apparatus according to claim 12 wherein said actuation mechanism is a foot pedal and the other end of said arm is spring loaded for moving said cap away from the substrate when said foot pedal is released.
 14. Apparatus according to claim 10 wherein said third means comprises an eccentric cam for moving said second shank in a substantially vertical plane for moving said cap into contact with the underside of a substrate.
 15. Apparatus according to claim 10 wherein said third means comprises a solenoid for moving said second shank in a substantially vertical plane for moving said cap into contact with the underside of a substrate.
 16. Apparatus according to claim 19 wherein said third means comprises hybraulic means for moving said second shank in a substantially vertical plane for moving said cap into contact with the underside of a substrate.
 17. Apparatus according to claim 10 wherein said third means comprises a pneumatic mechanism for moving said second shank in a substantially vertical plane for moving said cap into contact with the underside of a substrate.
 18. Apparatus according to claim 10 wherein said third means comprises a control circuit responsive to selective actuation thereof for moving said cap into contact with the underside of the substrate for a predetermined time interval and automatically withdrawing said cap from the substrate after said time interval is over.
 19. Apparatus for operating in conjunction with a source of energy for reworking a hybrid circuit having a discrete components solder mounted on one side of a substrate by performing one of the operations of removing a discrete component from or attaching it to the substrate, said apparatus comprising: heater assembly means comprising a cap and a shank that are made of material having good thermal conductivity characteristics, said cap having a bore therein receiving one end of said shank in a heat-transferring relationship, said cap being made of a metal that is chemically less active than the metal from which said shank is made for maintaining a thin film of oxide on the surface of said cap; first means for communicating said heater assembly means with the energy source for heating said cap; second means for supporting the substrate with the other side thereof, that is opposite the one side of the substrate, being adjacent said cap; and third means cooperating with said heater assembly means and said second means for providing relative movement of said substrate and said cap for causing a portion of said cap surface and an area on the other side of the substrate to come into contact with each other for heating through the substrate solder on the other side thereof.
 20. Apparatus according to claim 19 wherein the portion of said cap surface contacting the other side of the substrate is shaped to provide both conduction and radiation heating of a region of the substrate that extends over a surface area thereof that is greater than the contact area on the other side of the substrate.
 21. Apparatus according to claim 20 wherein said third means moves said cap into contact with the other side of the substrate. 